Since the proposed method is an instance of the CART algorithm, finding the optimal split for a categorical variable with \(q\)  levels generally involves checking \(2^q\)  subsets.
This becomes computationally infeasible for high values of \(q\) .
It remains an open question for future work if this can be sped by an optimal procedure as in regression with L2 loss [1]} and binary classification [2]} or by a clever heuristic as for multiclass classification [3]}.
